Ventilator



July 3, 1934. E. DIETZE ET AL VENTILATOR Filed Sept. 14, 1935 Jlructurevo. 6'

Value ofConslants Patented July 3, 1934 UNITED STATES VENTILATOREginhard Dietze, Westfield, N. J., and Walter Deming Goodale, Jr.,Brooklyn, N. Y., assignors to American Telephone and Telegraph Company,a corporation of New York Application September 14, 1933, Serial No.689,484 3 Claims. (oi. 98-94) This invention relates to windowventilators and more particularly to those designed to exclude streetnoise.

The object of this invention is to provide a 5 window ventilator whichis compact yet capable of attenuating outside noises to a high degree inits air passages while permitting the fiow of a large volume of air.

A feature of this invention is the inclusion in a window ventilator ofthis kind of an acoustic filter together with a duct having a. lining ofsound absorbing material and containing turns or baffles.

In the drawing, Figure 1 shows a cross-section taken horizontallythrough a window ventilator incorporating the present invention and Fig.2 shows a cross-section taken vertically through the same ventilator.The section lines on one figure indicate the plane of the section in theother figure. Fig. 3 shows the theoretical type of filter section usedin the window ventilator.

In window ventilators designed up to the present time the use of soundabsorbing material and baffles to reduce the transmission of noisethrough its air passages has been common. The

advantages to be derived from the combination of an acoustic filter withabsorbing material and battles reside in the characteristics of thesetwo methods of attenuating sound in the air passages.

The characteristics complement each other, the filter being mosteffective at frequencies where the absorbing material and bafllesordinarily provide little noise reduction, and the absorbing materialand battles being most effective at frequencies where the noiseattenuation of the filter is small.

Sound absorbing materials are generally most efficient at frequenciesabove 500 cycles per second. Un ess the material is very thick, in theorder of three to six inches, its absorbing efficiency for frequenciesbelow 150 cycles per second is approximately one-third that of 1000cycles per second. A duct or air passage lined with sound absorbingmaterial will, therefore, permit the passage of a greater portion oflowfrequency sound energy than high-frequency sound energy unless verythick materials are used. In order that a window ventilator providesatisfactory reduction of street noise, it should attenuate as wide arange of frequencies as possible, since noise usually contains soundenergy over a band of frequencies ranging from about cycles to 8000cycles or higher.

In ducts, the use of turns or bafiles covered with sound absorbingmaterial further assists in attenuating the higher frequencies more thanthe lower frequencies. Due to the laws of propagation of sound waves,baffles or turns absorb effectively only when the dimensions of thebafoo -fie or the cross-sectional dimensions of the turn are largecompared to the wave-length of the sound wave. From the foregoingdiscussion it may be seen that it is difiicult to construct a compactwindow ventilator, having passages lined with sound absorbing materialalone which will effectively attenuate low-frequency components ofoutside noise.

While it is possible to design acoustic filters to attenuate any desiredfrequency range, there are decided limitations in the atttziuation rangeof an acoustic filter that can be used in a window ventilator whose mainbranches must have a large area in order to pass a large volume of air.It has been found that a filter of this type becomes ineffective forhigher frequencies whose wave length is less than the cross-sectionaldimensions of the. main branches.

In a window ventilator the most desirable type of acoustic filter is alow-pass structure whose dimensions may be computed from the followingformulas taken from Table II, Column I, of an article entitled TheApproximate Networks of Acoustic Filters" by W. P. Mason in the Journalof the Acoustical Society of America, January, 1930.

where c=velocity of souncl=34,000 cm./sec.

density of air=0.001205 gram./cm.

zo characteristic impedance.

f1, I2, I are critical frequencies shown in the drawing labeledAttenuation characteristic in Column 1 of Table II.

L, 1, S1, S2, are dimensions as shown in the drawing labeled Structurein Columns 1 and 2 of Table II.

r1=radius of cylinder whose cross section area is S1.

rz=radius of cylinder whose cross section area is S2.

A more practical side branch construction is given in structure 5, TableI, of the above paper and for simplicity is reproduced in Fig. 3 of thepresent drawing. This construction is used in the filter incorporated inthe particular window ventilator described herein. The design formulasare:

where r1, n, t and T are dimensions as shown in the figure accompanyingstructure 5, Table I.

The above design formula presuppose an infinite structure. In order toobtain the same characteristics in a finite structure, it is necessary,therefore, to terminate the filter in its characteristic impedance, thatis, the acoustic impedance at the sound output end of the main passage(as distinguished from the air output end, although they may beidentical) should be equal to the characteristic impedance of the filterat all frequencies. When the sound output end of the passage terminatesin free air, this condition does not exist. An exponential horn of thecorrect dimensions may be connected to the passage to provide thecorrect termination. For frequencies in the order of 100 cycles persecond, the area of such a horn at the mouth must be over five squarefeet and for lower frequencies it must be greater. This is too large fora compact ventilator design. When, however, a duct having turns orbaiiles lined with an eificient sound absorbing material is connected tothe sound output end of an acoustic filter, the attenuationcharacteristics are improved by the acoustic load impedance of the duct,Furthermore, the high frequencies that pass through the filter areattenuated by the absorbing material. The combination of filter andlined duct thus attenuates sound energy over a wide range offrequencies. The use of baflles at the filter opening to improve theperformance of the filter by increasing the acoustic impedance intowhich the filter works constitutes another valuable feature of thisinvention.

In the particular design shown, the window ventilator 1 is mounted on aninside window sill 2. The lower window sash 3 is closed down on top ofthe outer vertical side 4 of the ventilator. The ventilator is fastenedto the window frame 5 by means of a strip of metal 6 sliding jointly inthe sash runway 7 and the slot 8 formed by the metal channel 9, which isfastened to the outer vertical side 4 of the ventilator. Air is drawn inthrough the outside opening 10 by the suction of the fan 12 beingdirected around the turn in the duct by vanes 11 as is common in ductshaving sharp turns, and is forced through the cylindrical shapedlow-pass acoustic filter 13.

The filter is built in two sections A and B. In accordance with commonpractice, section A has a lower cutoff frequentcy than section B so thatthe combination results in a better attenuation characteristic than twoidentical sections in tandem. Each section consists of tubes enteringeither end of a cylindrical chamber of larger dimensions, as shown inFig. 3. For example, section A includes tubes 16 and 1'7 enteringcylindrical chamber 14, while section B includes tubes 17 and 18entering the still larger cylindrical chamber 15. The truncatedconicalshape of the main branches 16, 17 and 18 is such that air at highvelocity will pass smoothly from one branch to the next in the directionindicated by the arrows, in accordance with aerodynamical theory. Thetransverse vanes 26 in the main branch 16 prevent any air turbulence inthe filter due to the rotary motion imparted by the fan. After leavingthe main branch 18 the air expands into the duct 20, being directed byvanes 19. As shown in Fig. 2, the air then passes around an expandingturn covered with absorbing material 22 on the far side and having vanesor baffles 21 of sound absorbing material, which prevent turbulence inthe air fiow and absorb noise. The velocity of the air is furtherreduced by expansion in the upper duct 23 to a point where there will beno appreciable air hiss at the outlet grille 25. Vanes or bellies 24 ofsound absorbing material, whose curvature is reverse to that of thebaiiies 21, distribute the air uniformly along the outlet grille 25 andfurther attenuate the noise. The batlies, above described, increase theacoustic impedance into which the filter works and thereby improve theattenuation characteristics, as previously stated.

The absorbing material 27 is provided throughout and rubber mountings 28are also provided, as shown on the drawing, to reduce the transmissionof vibrations.

While the above description relates to a particular construction, it isto be understood that various modifications may be made in the designwithout departing from the scope of the invention as defined in theappended claims.

What is claimed is:

1. A ventilating device including an intake, means to draw air throughsaid intake and force it through the ventilating device, an acousticalfilter on the output side of said last-mentioned means, and anopen-ended expanding chamber provided with sound absorbing battles onthe output side of said filter.

2. A ventilating device including an intake, means to draw air throughsaid intake and force it through the ventilating device, an acousticalfilter on the output side of said last-mentioned means, an open-endedexpanding chamber on the output side of said filter, said expandingchamber being curved longitudinally and of gradually increasingcross-section as the outlet is approached, and curved vanes of soundabsorbing material so positioned as to direct the air toward the openingand at the same time exercise a baiiling effect for the sound.

3. A ventilating device including an intake, means to draw air throughsaid intake and force it through the ventilating device, an acousticalfilter on the output side of said last-mentioned means, an open-endedexpanding chamber on the output side of said filter, said expandingchamber being developed in the form of a reverse curve and havinggradually increasing crosssection as the output is approached, andcurved vanes of sound absorbing material arranged at each turn of theexpanding chamber to direct the air and exercise a baffling effect forthe sound.

EGINHARD DIETZE. WALTER DEMING GOODALE, JR.

